It is known that ethylene copolymers having a density equal to that of high-pressure polyethylene may be obtained by copolymerization of ethylene and small amounts of .alpha.-olefin in the presence of a Ziegler catalyst. Generally, by reason of the fact that the polymerization is easy to operate, it is of advantage to adopt high temperature solution polymerization wherein the polymerization is carried out at a temperature higher than a melting point of the resulting copolymer in the presence of a hydrocarbon solvent. In that case, however, when polymers having a sufficiently high molecular weight are intended to obtain, the concentration of polymer in the polymerization solution must be made lower, because a viscosity of the solution increases, and accordingly there is such a problem that the productivity of copolymer per polymerizer unavoidably becomes low.
When low density ethylene copolymers are intended to obtain by slurry polymerization often used for preparing high density polyethylene, there were such problems that the resulting copolymer tends to dissolve or swell in the polymerization solvent, and not only the concentration of the slurry cannot be increased by such reasons as increase in viscosity of the polymerization solution, attachment of the resulting polymer to the wall of polymerizer and decrease in bulk density of the resulting polymer but also no long-term continuous polymerization operation becomes possible. There was also a problem from the standpoint of quality, because the copolymers obtained were found to be sticky. With the view of solving such problems as mentioned above, there have been proposed some methods wherein specific catalysts are used and prepolymerization is carried out.
In preparing low crystalline copolymers of ethylene and .alpha.-olefin, on one hand, there have heretofore been proposed many attempts to improve catalyst activities. For instance, such attempts include a method in which vanadium compounds excellent in copolymerizability are supported on carriers, a method in which oxidizing reagents are added to catalysts so as to improve activities of the catalysts, and a method for improving copolymerizability of carrier-supported titanium compounds high in activity. In these methods, however, the polymerization activity attained was still low, and the copolymerizability exhibited was not found to be sufficient, thus a further improvement was desired.
Japanese Patent L-O-P Publn. No. 195108/1985 discloses solid titanium catalyst components for olefin polymerization, which are obtained by reaction of a hydrocarbon-insoluble solid magnesium aluminum composite selected for the following (A.sub.1) and (A.sub.2) with a tetravalent titanium compound, said solid titanium catalyst components containing at least 10%, based on the total amount of titanium atoms, of titanium atoms in a low valence state, and 1-15 OR groups (R is a hydrocarbon group) in terms of OR/Mg (weight ratio).
(A.sub.1) A solid magnesium aluminum composite having a R.sup.1 O group and a reducing R.sup.2 group (R.sup.1 and R.sup.2 are each a hydrocarbon group), said composite being obtained from a magnesium compound in the liquid state formed from a mixture containing a magnesium compound and an electron donor, or from a magnesium compound in the liquid state formed from a hydrocarbon solvent solution of a magnesium compound. PA0 (A.sub.2) A solid magnesium aluminum composite having a R.sup.1 O group and a R.sup.3 O group (R.sup.3 is a hydrocarbon group), said composite being obtained by reaction of a solid magnesium compound (B.sub.1) containing the R.sup.1 O group but containing no reducing R.sup.2 group, obtained from a liquid magnesium compound formed from a mixture containing a magnesium compound and an electron doner or from a liquid magnesium compound formed from a hydrocarbon solvent solution of a magnesium compound, a solid magnesium compound (B.sub.2) containing the R.sup.1 OH group, obtained from a liquid magnesium compound, or the above-mentioned (A.sub.1) with an organometallic compound (C) of a metal belonging to the Group 1 to 3 of the periodic table.
An object of the present invention is to provide titanium catalyst components which exhibit excellent slurry polymerizability when used in polymerization of homopolymer of ethylene or in the case of preparation of low density ethylene copolymers by copolymerization of ethylene and .alpha.-olefin, which are readily applicable to vapor phase polymerization to prepare copolymers having a narrow composition distribution, with the result that the resulting low density ethylene copolymers can be molded into molded articles, such as film, excellent in transparency, antiblock properties and heat sealability and even in a process such as vapor phase polymerization wherein all the resulting copolymers are formed into articles of manufacture, such excellent molded articles as above can be obtained, and which are high in efficiency of utilizing starting materials when catalysts are prepared therefrom. And accordingly the subsequent waste liquid treatment becomes easy, and processes for preparing said titanium catalyst components.
Another object of the invention is to provide catalysts containing such titanium catalyst components as mentioned above for use in the preparation of ethylene polymers, and processes for preparing said ethylene polymers by using said catalysts.